1. Field of the Invention
The present invention relates to a display device using an organic light-emitting device that has an anode, a cathode, and a film containing an organic compound that emits light by application of electric field (hereinafter referred to as organic thin film). Specifically, the present invention relates to a high reliable display device that drives at a low voltage. The organic thin film contains an organic compound as light-emitting compounds, and also may contain an inorganic compound as another component elements. The term display device in this specification refers to an image display device that employs the organic light-emitting device as a light-emitting device. Also included in the definition of the display device are: a module in which a connector, such as an anisotropic conductive film (FPC: flexible printed circuit), a TAB (tape automated bonding) tape, or a TCP (tape carrier package), is attached to the organic light-emitting device; a module in which a printed wiring board is provided on the tip of a TAB tape or a TCP; and a module in which an IC (integrated circuit) is mounted directly to the organic light-emitting device by the COG (chip on glass) method.
2. Description of the Related Art
The organic light-emitting device emits light when electric field is applied. Light emission mechanism thereof is said to be as follows. A voltage is applied to an organic thin film sandwiched between electrodes to cause recombination of electrons injected from the cathode and holes injected from the anode in the organic thin film and, the excited molecule (hereinafter referred to as molecular exciton) emits light resultingly with releasing energy when returns to base state.
There are two types of molecular excitons from organic compounds; one is singlet exciton and the other is triplet exciton. This specification includes both cases where singlet excitation causes light emission and where triplet excitation causes light emission.
In the organic light-emitting device such as the above, its organic thin film is usually formed to have a thickness of less than 1 μm. In addition, the organic light-emitting device does not need back light that is required in conventional liquid crystal displays since it is a self-light-emitting device in which light is emitted from the organic thin film by itself. Therefore the great advantage of the organic light-emitting device is very thin and light-weight.
When the organic thin film having a thickness of about 100 to 200 nm, for example, recombination takes place within several tens nanoseconds after injecting carriers, based on the mobility of the carriers in the organic thin film. Considering the process from carrier recombination to light emission, the organic light-emitting device is readied for light emission in microseconds. Accordingly, quick response is also one of the advantages of the organic light-emitting device.
Since the organic light-emitting device is of carrier injection type, it can be driven with a direct-current voltage and noise is hardly generated. Regarding a driving voltage, a report says that a sufficient luminance of 100 cd/m2 is obtained at 5.5 V by using a super thin film with a uniform thickness of about 100 nm for the organic thin film, choosing an electrode material capable of lowering a carrier injection barrier against the organic thin film, and further introducing the hetero structure (two-layer structure) (Reference 1: C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes”, Applied Physics Letters, vol. 51, no. 12, 913–915 (1987)).
It can be said that the organic light-emitting device demonstrated in Reference 1 is characterized by separation of functions of the hole transporting layer and the electron transporting light-emitting layer in which the former layer is assigned to transport holes and the latter layer is assigned to transport electrons and emit light. The idea of separation of functions has been developed to a double hetero structure (three-layer structure) in which a light-emitting layer is sandwiched between a hole transporting layer and an electron transporting (Reference 2: Chihaya ADACHI, Shizuo TOKITO, Tetsuo TSUTSUI, and Shogo SAITO, “Electroluminescence in Organic Films with Three-Layered Structure”, Japanese Journal of Applied Physics, vol. 27, No.2, L269–L271 (1988)).
An advantage of separation of function is an expansion of freedom in molecular design (for example, it makes unnecessary to make the effort to find bipolar materials) since it is unnecessary to give simultaneously various functions (luminescence, carrier transportation, and carrier injection from electrodes) to one kind of organic material. In other words, high luminescent efficiency can be obtained easily by combining materials excellent in luminescent characteristics with materials excellent in carrier transportation ability.
With respect to separation of function, conception of a cathode buffer layer and an anode buffer layer is suggested as an introduction of a function of carrier injection to reduce driving voltage. There is a report that the driving voltage is reduced by enhancing injection of carrier by means of inserting materials that ease energy barrier into an interface between cathode and the organic thin film thereto (Reference 3: Takeo Wakimoto, Yoshinori Fukuda, Kenichi Nagayama, Akira Yokoi, Hitoshi Nakada, and Masami Tsuchida, “Organic EL Cells Using Alkaline Metal Compounds as Electron Injection Materials”, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 44, NO. 8, 1245–1248 (1997)). In Reference 3 it is disclosed that Wakimoto et al. succeeded to reduce driving voltage by using Li2O as a cathode buffer layer.
With respect to a buffer layer, a buffer layer comprising polymer attracts especially attention in recent years (Reference 4: Yoshiharu Sato, Molecular Electronics and Bioelectronics (The Japan Society of Applied Physics), vol. 11, No.1, 86–99 (2000)). In Reference 4 it is disclosed that using an anode buffer layer comprising polymer promotes the lower voltages, longer lifetime, and higher heat resistance. The anode buffer layer comprising polymer can be formed thick since the conductivity is increased by introducing appropriate accepter. Thus, it can contribute to flatness and is expected that it have an effect on decreasing short circuit.
With those features, including being thinner and lighter, quick response, and direct current low voltage driving, the organic light-emitting device is attracting attention as a next-generation flat panel display device. In addition, with being a self-light-emitting type and a wide viewing angle, the organic light-emitting device has better visibility and is considered as effective especially in using for a display screen of in-car products and portable equipments. Practically, the organic light-emitting device is used for a display screen of area color of in-car audio equipments.
Another feature of the organic light-emitting device is emission of light of various colors. The well varied colors are derived from the diversity of organic compounds of its own. In other words, the various colors are derived from the flexibility, with which materials emitting different colors can be developed by designing a molecule (introduction of a substituent, for example).
From these points, it is safe to say that the most promising application field of organic light-emitting devices is in full color flat panel displays without mentioning mono color and area color displays. Various methods have been devised to display full color while considering the characteristics of organic light-emitting devices. Currently, there are three major methods for manufacturing a full color display device using the organic light-emitting device. One of those major methods is to separately form the organic light-emitting device that emits red light, the organic light-emitting device that emits green light, and the organic light-emitting device that emits blue light using a shadow mask technique. Red, green, and blue are the primary three colors of light, and each of the three types of organic light-emitting devices makes one pixel. This method is hereinafter referred to as an RGB method. Another one of the major methods obtains the primary three colors of light by using a blue organic light-emitting device as a light emission source and converting the blue light into green light and red light through color conversion layers that are formed of organic fluorescent materials. This method is hereinafter referred to as a CCM method. The last one is a method of obtaining the primary three colors of light by transmitting white light from a white organic light-emitting device used as a light emission source through color filters that are used in liquid crystal display devices or the like. This method is hereinafter referred to as a CF method.
In any of these configuration, driving methods such as passive matrix driving (simple matrix type) and active matrix driving (active matrix type) are used for a display device that is formed by arranging the organic light-emitting devices as a matrix of pixels. In addition, in the case that the pixel density is thickened, it is said that the active matrix type provided switches (for example, non-linear elements such as transistors) in each pixel has an advantage over the passive matrix type because it can drive at a low voltage.
Meanwhile, as mentioned previously the buffer layer comprising polymer as demonstrated in Reference 4 promotes lower driving voltage, longer lifetime, and higher heat resistance. A problem has arisen when the organic light-emitting device having a buffer layer (mainly anode buffer layer) comprising these materials is tried to apply by arranging into matrix of each pixel in the display device. The problem is crosstalk.
In most buffer layer comprising polymer, donor or accepter is added to its polymer comprising π conjugated system to give them conductivity. The polymer is usually applied whole surface by spin coating and the like, and that leads to current leakage between polymer and wirings in places.
For example, it is reported that use of polyethylene dioxythiophene/polystyrene sulfonate (hereinafter referred to as “PEDT/PSS”) that is conductive polymer added with accepter as an anode buffer layer for forming a passive matrix display device causes crosstalk (Reference 5: A. Elschner, F. Jonas, S. Kirchmeyer, K. Wussow, “High-Resistivity PEDT/PSS for Reduced Crosstalk in Passive Matrix OELs”, Asia Display/IDW '01, 1427–1430 (2001)). In Reference 5 it is described that resistivity of PEDT/PCC is made high on purpose to avoid crosstalk.
However, if the resistivity is made high, the buffer layer comprising polymer can not be formed to have a thick film (that is, current does not pass easily through the organic light-emitting device). Therefore, the characterization that avoid short circuit due to flattening of electrode surface by means of making the film thickening is lost. High resistivity leads to high driving voltage spontaneously. Thus, the advantage of low driving voltage is also lost.